Tissue Discrimination and Applications in Medical Procedures

ABSTRACT

A system and method for discriminating tissue types, controlling the level of therapy to tissue, and determining the health of a known tissue by measuring the characteristics of an electrical signal applied to a conductive element located within or by the tissue. Additionally, the system and method may be used for determining whether the conductive tip of a pedicle probe or pedicle screw is located in one of cortical bone, cancellous bone, and cortical bone near a boundary with soft tissue, whether the conductive tip of a cannula is located adjacent to one of nerve tissue and annulus tissue, and whether the conductive tip of a cathode is located adjacent to one of nerve tissue and prostate gland tissue.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/609,004 entitled “Tissue Discrimination and Applications inMedical Procedures,” filed Oct. 29, 2009, which is a continuation ofU.S. patent application Ser. No. 11/439,780 entitled “TissueDiscrimination and Applications in Medical Procedures,” filed Mar. 29,2004, which is a divisional of U.S. patent application Ser. No.10/812,038, entitled “Tissue Discrimination and Applications in MedicalProcedures,” filed Mar. 29, 2004 (issued as U.S. Pat. No. 7,050,848),which is a divisional of U.S. patent application Ser. No. 09/860,648,entitled “Tissue Discrimination and Applications in Medical Procedures,filed May 18, 2001 (issued as U.S. Pat. No. 6,760,616), which claimsbenefit under 35 U.S.C. §119(e) from U.S. Provisional Application Ser.No. 60/205,634 filed May 18, 2000 and U.S. Provisional Application Ser.No. 60/243,465 filed Oct. 25, 2000, the entire contents of which areeach hereby expressly incorporated by reference into this disclosure asif set forth fully herein.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention is related to tissue surveillance systems.

II. Discussion of the Prior Art

Systems and methods exist for determining when a probe, needle, catheteror other devices make contact with a particular tissue, e.g. U.S. Pat.No. 5,836,990 to Li entitled “Method and Apparatus for DeterminingElectrode/Tissue Contact.” The Li patent teaches a method fordetermining when a catheter makes contact with tissue covered with anionic liquid. The system measures the electrical impedance at a distalend of the catheter and determines tissue contact has been made when theimpedance increases. The system does not identify the type of tissuecontacted and presumes the tissue is covered in an ionic liquid.Accordingly, a need exists for a system and method that identifiestissue and uses this information in medical procedures.

Systems and methods also exist for controlling the level of ablation oftissue. These systems monitor the impedance of tissue being ablated todetermine if the ablation energy is optimal. The systems generallymeasure impedance to within approximately 20 ohms. These systems do notdetermine when sufficient therapy has been applied to the tissue oremploy impedance measurement with low tolerance levels. Accordingly, aneed exists for a system that may control any form of therapy bymonitoring characteristics of an electrical signal applied to thetissue.

SUMMARY OF THE INVENTION

The present invention provides a system in which an electrical signal isapplied to a tissue via electrodes disposed on a tissue probe. Theelectrical signal applied to the tissue preferably comprises a frequencyvariable current or voltage that is preferably applied to the tissueusing a sliding frequency scale.

In accordance with the present invention, the response to the appliedsignal is measured as the signal passes through tissue disposed at,around, or adjacent to the probe. The inventors have found thatdifferent tissue types display different electrical transmissionproperties, including different capacitance and impedance properties.Accordingly, by measuring the electrical characteristics of the responsesignal, it is possible to determine the type of tissue through which thesignal is passing. Preferably, this is accomplished by comparison toknown exemplary signal characteristics for various tissue types.Furthermore, when the probe is known to be disposed within, around, oradjacent to a first tissue, the system and method may determine when theprobe is advanced into a second tissue based on the changed electricalcharacteristics of the signal applied by the probe.

In accordance with the present invention, the electrical signalcharacteristics that are monitored may include the phase shift betweenthe voltage and current passing through a selected tissue, and theimpedance of the selected tissue. The inventors have experimentallydetermined that these properties vary from one tissue type to another.In a preferred aspect of the present invention, the electrical signalapplied to the tissue may be a sliding frequency signal such that afrequency spectrum of phase shift and impedance of a tissue isdetermined. However, any electrical, magnetic, or optical signal with ameasurable phase relationship and impedance to passage through thetissue can be used.

In a preferred method, a probe is advanced to a position in, at, oradjacent to a selected tissue and an electrical signal is applied to thetissue by an electrode on the probe. The response to this signal is thenmeasured and compared against known electrical, magnetic, or opticaltransmission characteristics for the various tissue types. For example,the present invention provides a method and system for determiningwhether the conductive tip of a pedicle probe or pedicle screw islocated in one of cortical bone, cancellous bone, and cortical bone neara boundary with soft tissue, whether the conductive tip of a cannula islocated adjacent to one of nerve tissue and annulus tissue, and whetherthe conductive tip of a cathode is located adjacent to one of nervetissue and prostate gland tissue.

Furthermore, the inventors have discovered that the signal transmissioncharacteristics of various tissues vary as a function of the tissue'shealth. Accordingly, the present system can also be used to determinetissue health (for various tissue types) by comparing the signalresponses of tissue in response to stimulation by the probe to responsesfor healthy tissue.

The inventors have also determined that different cell/tissue typesexhibit different capacitive effects. In addition, these capacitiveeffects vary considerably between living and dead cells. Accordingly, inanother aspect of the invention, the present system discriminatesbetween living and dead tissues. This feature of the invention isparticularly useful when the present system is used in conjunction witha tissue ablation system. For instance, the tissue ablation system maybe prevented from providing unnecessary energy to ablate tissue andthereby protect surrounding tissue.

Moreover, the present system can be adapted to sense the presence of aparticular type(s) of tissue as the probe is advanced through thepatient's body. Such a feature is particularly advantageous when sensingfor the presence of nerve tissue. Specifically, the probe can beadvanced through the patient's body while the response to the electricalstimulation emitted by the probe is continuously monitored such that asnerve tissue is approached, the response signal will begin to exhibitcharacteristics indicative of nerve tissue.

Such nerve sensing features of the present invention can be used, forexample, to sense for the presence of spinal nerves when advancingsurgical equipment (which by way of example only may include cutting,drilling, screw insertion, implant, and tissue ablation systems) towardsthe patient's intervertebral space.

In an optional aspect of the present invention, a probe having anelectrode positioned thereon is replaced with a probe that is itselfelectrified. For example, an electrified needle or an electrified trocaror cannula can be used as the probe. An advantage of having the entireprobe emit the signal (rather than just an electrode disposed thereon)is that the probe itself can be made to smaller dimensions, particularlyin the case of an electrified needle.

In optional aspects of the present invention, the probe is mono-polar.Specifically, only a first electrode is disposed on the probe. A secondelectrode is then positioned some distance away from the first electrodeat another location on the body. Alternately, the probe may be bi-polarwith both the first and second electrodes positioned on the probeitself. Additionally, the probe may include a plurality of bi-polarelectrodes placed along the probe (such as around the tip and the lengthof the probe) to determine tissue types around the probe.

In a preferred aspect of the present invention, the measurement of thephase angle relationship between the voltage and current of the signaland impedance of the signal may be used to determine: (1) the type oftissue in which the probe is located; (2) the health of the tissue; (3)the relative location of the tip of the probe (e.g. in cases where theelectrode is disposed in the tip of the probe); or (4) any combinationof (1), (2) and (3). As such, by gathering data mapped by analyzing theresponse signal, measured characteristics can be used to correlate: (1)tissue identity, (2) tissue health, and (3) tissue location.

In addition, the present invention can be adapted to: (5) locatespecific tissue within a body; (6) control application of therapy totissue; (7) detect the state of health of a tissue; (8) navigate totissue; and (9) any combination of the above-mentioned applications.

In one embodiment, the invention is a tissue system including a computersystem having an analog- to-digital (A/D) converter and adigital-to-analog (D/A) converter interface (PCI board), that may beused to generate the control signal which is applied to the electrode orconductive tip of the probe. The computer generates the signal via theD/A converter. Then the A/D converter converts the signal received fromthe conductive tip into digital samples by sampling the signal at apredetermined rate where the digital samples may have a fixed orvariable number of bits and have linear, logarithmic or other scaling.The computer system determines characteristics of the received signalfrom the digital samples, in particular the phase angle and impedance atthe conductive tip or other location of the probe where the electrode(s)may be located. Based on the determined characteristics taken over time(which are then stored in a knowledge base or tabulated form), thepresent invention may determine tissue identity and tissue location. Ina preferred aspect, the electrode disposed on the probe comprises abipolar electrode conductive tip probe.

In an optional aspect of the present invention, the application oftherapy to the tissue in which the probe is located may be preciselycontrolled, based on the characteristics of the tissue where the probeis located. For example, the application of heating or cooling therapymay be used to ablate or cool tissue. In one exemplary aspect, the sameelectrode(s) used for tissue discrimination (e.g. determining tissuetype for tissue disposed adjacent to the electrode on the probe) mayalso be used for tissue ablation by heating.

In various aspects, the level of heating or cooling of the tissue may bemodulated as a function of the measured characteristics of the tissue.In particular, the phase angle and impedance of the tissue change as thetissue is heated or cooled to a certain level. Accordingly, theapplication of therapy may be regulated by the present computer system.In particular, the computer system may communicate with a deviceapplying therapy so as to automatically control the level of therapy.

Given that the present system can determine the type and location ofvarious tissues within a patient, the present system may be used todetermine the relative health of the tissue. In particular, the measuredcharacteristics of the signal will vary for diseased or unhealthytissue, as compared to normal healthy tissue. Thus, the present systemmay be used not only to determine the type and health of tissue, butalso to control therapy for tissue based on the same. Furthermore, theprobe may optionally be coupled with an automated navigation system thatnavigates within the patient based on the measured characteristics ofthe received signal. Such a navigation system may use the tissueidentity and location data to navigate to a particular location withinan organ. The computer system may then determine the health of thetissue at the location within the organ and control the application oftherapy as appropriate.

As can be envisioned by one of ordinary skill in the art, many differentcombinations of the above features of the present invention may be used.Accordingly, the present invention is not limited by the scope of theappended claims.

In optional aspects of the invention, the characteristic electricalproperties of the various tissue types are determined for differenttissues at different RF frequencies. By way of example only, the signalmay be emitted from the probe (into the surrounding tissue) atfrequencies in the range of 400 kHz to 100 MHz. Determining theelectrical properties of various tissues at various signal frequenciesmay be advantageous in that different cell (e.g. tissue) types mayexhibit different harmonics. As such, tissues may be furthercharacterized by measuring phase shift or impedance at variousfrequencies, or along a sliding frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

Many advantages of the present invention will be apparent to thoseskilled in the art with a reading of this specification in conjunctionwith the attached drawings, wherein like reference numerals are appliedto like elements and wherein:

FIG. 1 is a block diagram of a tissue discrimination system inaccordance with the present invention;

FIG. 2 illustrates a method of controlling the application therapy totissue according to the present invention; and

FIG. 3 illustrates a method of determining tissue health according tothe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure. The tissue discrimination system and related methodsdisclosed herein boasts a variety of inventive features and componentsthat warrant patent protection, both individually and in combination.

FIG. 1 is a diagram of a tissue identification system 10 in accordancewith the present invention. The system 10 includes a user-readableoutput device 12, a user input device 16, a processor 20, and a probe22. The processor 20 includes a central processing unit (“CPU”) 14 andDigital-to-Analog Converter (“D/A”) and Analog-to-Digital (“A/D”)Converter 18. The CPU 14 may comprise any microprocessor havingsufficient processing power to control the operation of the D/A & A/D 18and output device 12. The D/A & A/D 18 may comprise any such devicehaving a sufficient operating cycle to generate signals with thefrequencies described herein and sufficient sampling rate to generatethe digital samples described herein. The probe 22 may comprise anymedical device that may be used to hold one or more electrodes thereonwhere the electrodes transmit and receive electrical signals. By way ofexample only, the probe 22 may consist of any such suitable medicalinstrument including but not limited to a cannula, needle, catheter, FRablation device, or laser. The probe 22 may have a single electrode(mono-polar), two electrodes (bipolar), or a plurality of electrodes(multi-polar) configuration. Throughout the remainder of the discussion,a probe 22 with a conductive tip is discussed as one exemplaryembodiment. It is understood that the electrodes could be placedanywhere along the circumference or width and length of the probe 22. Aprobe 22 having multiple electrodes ideally includes groups of bipolarelectrodes so the system or method of the present invention may map theresponse of the electrode pairs.

The CPU 14 controls the operation of the D/A & A/D 18 and output device12 based upon user selection received via the user input device 16. Theuser input device 16 may comprise any input device including but notlimited to a keyboard, mouse, or touch-sensitive screen. The outputdevice 12 may consist of any output device controllable by the CPU 14including but not limited to a computer monitor, printer, or othercomputer controlled display device. The system 10 generates anelectrical signal that is transmitted to tissue near or about the probe22. When the probe 22 has an omni-directional conductive tip, theelectrical signal may be propagated to a wide area of tissue about theconductive tip. The conductive tip may include an electrode pair(bipolar) so that the electrical signal is directed primarily to tissuedirectly in the path of the probe's 22 conductive tip (electrode pair).The system 10 provides an electrical signal at the electrode(s) on theprobe 22 via the D/A 18. In particular, the CPU 14 generates a digitalrepresentation of a signal to be transmitted by the probe 22. The D/A 18converts the digital signal to an analog signal that is transmittedthrough tissue by the probe 22.

The probe 22 also receives signals conducted by tissue surrounding theconductive tip of the probe 22. The A/D 18 converts the analog signalreceived by the electrode(s) of probe 22 into a digital signal that maybe processed by the CPU 14.

In one embodiment, the system 10 applies a fixed frequency signal to theelectrodes located on probe 22. In an exemplary embodiment, the appliedsignal may have a frequency ranging from 400 KHz to 100 MHz. The system10 may further apply a signal having a range or sliding frequency. Thesystem 10 applies this signal to the electrode(s) via the CPU 14 and D/A18.

The repeatable pattern of the applied signal may be any pattern wherethe phase of the signal may be determined, e.g. any signal having ameasurable phase relationship (voltage to current). In one embodiment,the applied signal may be sinusoidal in nature. In an alternativeembodiment, the signal may be a square wave signal where the phase ofthe signal is measured at a leading or a trailing edge of each squarewave. As previously stated, any signal with a measurable phaserelationship (voltage to current) may be used.

The A/D 18 converts signals received at the electrode(s) of the probe 22to a digital signal for processing by the CPU 14. The CPU 14 determinescharacteristics of the tissue surrounding the probe's 22 electrodes bycomparing the signal applied to the electrode(s) and the signal receivedfrom the same. In one embodiment the phase angle between voltage andcurrent of the applied signal (effective capacitance) and impedance ofthe tissue surrounding the conductive element (electrode(s)) of theprobe 22 is determined. It has been found that the measurement of thephase angle relationship and impedance may be used to determine theidentity or type of tissue in which the probe electrode(s) is located,the relative health of the tissue, the relative location of theelectrodes to other surrounding tissue, and to control the applicationof therapy to the tissue surrounding the probe's 22 electrode(s). In oneembodiment, the measured characteristics may be used to determine theidentity or type of tissue in which the probe electrode(s) is located,the relative health of the tissue, the relative location of the tip toother surrounding tissue, and to control the application of therapy tothe tissue surrounding the probe's 22 electrode(s).

For example, the probe 22 may be placed in the kidney of a patient.Then, system 10 may apply a signal to the probe's 22 electrode(s) havinga varying or fixed frequency. Then the system 10 determines the phaseangle and impedance of the signal applied to the probe 22 for eachfrequency of the signal. In one embodiment, the system 10 may use thecombination of characteristics and frequency of the applied signal todetermine 1) that the electrode(s) of the probe is located within kidneytissue (identification of tissue) and 2) the specific location withinthe kidney tissue where the probe is located, e.g., near the outercordial or inner medulla of the kidney (specific identification of thetissue).

The system 10 may also determine whether the kidney tissue about theelectrode(s) of the probe 22 is healthy, e.g., ischemic, or has tumors.By first knowing that the electrode(s) is in kidney tissue (a firsttissue type), the system 10 can look for changes in the signalcharacteristics to determine that unhealthy tissue (a second tissuetype) is present within the kidney. When the system 10 determines thatthe tissue about the probe's 22 electrode(s) is not healthy, the system10 may apply therapy to the tissue. The therapy may include theapplication of heat energy (ablation) or removal of heat energy(cryogenic cooling) of the tissue. The system 10 may continue to monitorcharacteristics of the tissue about the electrode(s) to determine whensufficient therapy has been applied. Then, when sufficient therapy hasbeen applied, the system 10 may stop the application of therapy. In oneembodiment, sufficient therapy has been applied when the tissue dies.The system 10 may then monitor the phase angle and impedance of theapplied signal to determine when cell or tissue necrosis has occurred.The system 10 may also consider the frequency of the applied signalrelative to the phase angle and impedance.

The A/D converter 18 converts the signal received from the electrode(s)into digital samples by sampling the signal at a predetermined ratewhere the digital samples may have a fixed or variable number of bitsand have linear, logarithmic or other forms of scaling. The system 10determines characteristics of the received signal from the digitalsamples, in particular the phase angle and impedance at theelectrode(s). The system 10 may also include a knowledge base coupled tothe CPU 14. The knowledge base may be stored characteristics about alarge variety of known tissues. The base may also be correlated orindexed on the frequency of the applied signal. The knowledge base maybe a database stored in a fixed electronic medium (not shown) coupled tothe CPU 14. In this embodiment, the CPU 14 compares the determinedcharacteristics to characteristics stored in the database to determinetissue identity, location, health, and control the application oftherapy. It is noted that the invention may also know the currentposition of the electrode(s), e.g. in which tissue the electrode(s) iscurrently disposed. The knowledge base may further include informationthat correlates to known current position of electrode(s) (within afirst tissue) with measured characteristics so the system 10 maydetermine a second tissue type. Accordingly, the system 10 or method ofthe invention may determine the tissue type of a second tissue based onknowledge of the position of the electrode(s) in a previous first tissueand measured characteristics of the signal applied to the electrode(s).

It is noted that in another embodiment, the present invention may beused with a device that automatically navigates through tissue. By wayof example only, the present invention may be coupled to an automatedcatheter system. The system 10 would provide tissue identity andlocation to the navigation system so the navigation system may navigateto a desired location. Once at the desired location, the system 10 maydetermine the health of the tissue. Then, the system 10 may control theapplication of therapy to the tissue based on the determined health ofthe tissue.

For example, a navigation system used in conjunction with system 10 maydirect the probe 22 to a specific location within kidney tissue. Basedon the known characteristics of the tissue, the health of the tissue maybe determined and the application of therapy may be applied when needed.A method 30 of applying therapy is shown in FIG. 2. In step 32, themethod 30 first determines the initial characteristics of the tissue.Then therapy is applied to the tissue (step 34). Therapy for the kidneytissue may include the application of heat or cooling therapy to ablateor cool the tissue. The level of heating or cooling of the tissue may bemodulated as a function of the measured characteristics of the tissue.In particular, the phase angle and impedance of the tissue will changeas the tissue is heated or cooled to a certain level. The method 30applies a signal to electrode(s) in the tissue receiving therapy (step36). The method 30 then determines the current tissue characteristicsbased on the applied signal (step 38). When the desired tissuecharacteristics are present (sufficient therapy applied) (step 40), themethod 30 stops the application of therapy to the tissue (step 42). If,on the other hand, the desired tissue characteristics are not achieved,the method 30 returns to the step of applying a signal to theelectrode(s) in tissue (step 36) and again progresses through theremaining steps. Accordingly, the application of therapy may beregulated by the system 10. In one embodiment, the system 10communicates with a device applying therapy and automatically controlsthe level of therapy.

As noted, the system 10 may be used to determine the relative health ofa tissue. The measured characteristics of the signal will vary fordiseased or unhealthy tissue. For example, it has been found thatcancerous cells have measurably different impedance from that of healthytissue. FIG. 3 illustrates a method 50 of determining tissue healthaccording to the present invention. The method 50 places theelectrode(s) in known tissue (step 52). The tissue may be known by firstdetermining the location of the electrode(s) using techniques describedabove. Then the method 50 applies a signal to the electrode(s) in thetissue of interest (step 54). The signal may be a signal of varyingfrequency, e.g. a sliding frequency signal in one embodiment. The method50 then determines the tissue characteristics based on the appliedsignal (step 56). The determined characteristics are compared to normalor expected characteristics for health or normal known tissue (step 58).When the determined characteristics are different from the expectedcharacteristics for the known tissue (when healthy) (by some toleranceamount), the method 50 indicates that the tissue at the electrode(s) isunhealthy (step 62). The method 50 may also indicate what type ofdisease the tissue may have based on known characteristics of diseasedtissue e.g. tissue appears to be cancerous or ischemic. Otherwise, thesystem may report that the tissue near the electrode(s) appears to behealthy (step 64).

In another embodiment, the probe 22 may be a pedicle screw or pedicleprobe. During the insertion of a pedicle screw, it is critical that thepedicle wall is not violated. Surgeons use image intensifiers and otherequipment to prevent such a violation. The tissue discrimination system10 of the present invention may be used to monitor the position of thepedicle probe or pedicle screw. In particular, the system 10 monitorsthe impedance and capacitance or phase shift at the tip of the pedicleprobe or screw to determine whether the tip is in cortical bone,cancellous bone, or cortical bone near a boundary with soft tissue.

In this embodiment, the outer surface of the pedicle screw may benon-conductive except for the surface of the head and tip of the pediclescrew. Likewise, the outer surface of the pedicle probe isnon-conductive except for the distal and proximal ends of the probe. Aconductive lead is then applied to the head of the pedicle screw orproximal end of the pedicle probe to conduct a signal to the tip of thescrew or probe. In one embodiment, the signal may have a fixed orvarying frequency. The system 10 then determines the phase angle andimpedance of the signal applied to the tip for each frequency of thesignal. To accomplish this, the system 10 may use the combination ofcharacteristics and frequency of the applied signal to determine whetherthe tip is located in cortical bone, cancellous bone, or cortical bonenear the boundary with soft tissue. Depending on this determination, thesurgeon may continue the insertion of the pedicle probe or screw.

In another embodiment, the probe 22 may be a cannula to be insertedadjacent to an annulus of a patient's spinal disc prior to performing anannulotomy. During the insertion of the cannula towards the annulus, itis critical that the cannula not rest against a nerve along side theannulus wall. Surgeons use electromyography (EMG) equipment and otherequipment to prevent such a situation. The tissue discrimination system10 of the present invention may be used to monitor the position of thecannula as it is advanced to the annulus wall. In particular, the system10 monitors the impedance and capacitance or phase shift at the tip ofthe cannula to determine whether the distal tip is adjacent to nervetissue or annulus tissue.

In this embodiment, the outer surface of the cannula is non-conductiveexcept for the distal and proximal ends of the cannula. A conductivelead is then applied to the proximal end of the cannula to conduct asignal to the tip of the cannula, the signal having a fixed or varyingfrequency. The system 10 then determines the phase angle and impedanceof the signal applied to the tip for each frequency of the signal. Toaccomplish this, the system 10 may use the combination ofcharacteristics and frequency of the applied signal to determine whetherthe tip is located adjacent to nerve tissue or annulus tissue. Dependingon this determination, the surgeon may continue the insertion of thecannula.

In another embodiment, the probe 22 may be an ablation cathode to beinserted into a patient's prostate gland prior to performing prostategland ablation. During the insertion of the cathode into the prostategland, it is critical that the cathode is not near or adjacent to nervetissue along side or within the prostate gland. Surgeons use imageintensifier equipment and other equipment to prevent such a situation.The tissue discrimination system 10 of the present invention may be usedto monitor the position of the cathode as it is advanced into theprostate gland. In particular, the system 10 monitors the impedance andcapacitance or phase shift at the tip of the cathode to determinewhether the distal tip is adjacent to nerve tissue or prostate glandtissue.

In this embodiment, a signal is applied to the ablation cathode tip, thesignal having a fixed or varying frequency. The system 10 thendetermines the phase angle and impedance of the signal applied to thetip for each frequency of the signal. To accomplish this, the system 10may use the combination of characteristics and frequency of the appliedsignal to determine whether the tip is located adjacent to nerve tissueor prostate gland tissue. Depending on this determination, the surgeonmay continue the insertion of the cathode.

While this invention has been described in terms of the best mode forachieving this invention's objections, it will be appreciated by thoseskilled in the art that variations may be accomplished in view of theseteachings without deviating from the spirit or scope of the presentinvention. For example, the present invention may be implemented usingany combination of computer programming software, firmware, or hardware.As a preparatory step to practicing the invention or constructing anapparatus according to the invention, the computer programming code(whether software or firmware) according to the present invention willtypically be stored in one or more machine readable storage mediums suchas fixed (hard) drives, diskettes, optical disks, magnetic tape,semiconductor memories such as ROMs, PROMs, etc., thereby making anarticle of manufacture in accordance with the invention. The article ofmanufacture containing the computer programming code is used by eitherexecuting the code directly from the storage device, by copying the codefrom the storage device into another storage device such as a hard disk,RAM, etc. or by transmitting the code on a network for remote execution.

As can be envisioned by one of skill in the art, many differentcombinations of the above may be used and accordingly the presentinvention is not limited by the scope of the appended claims.

1. A method of determining whether a conductive element of a probe islocated within body tissue of at least one of cancellous bone, corticalbone, and cortical bone near a boundary with soft tissue, comprising thesteps of: (a) applying an electrical signal to a bi-polar electrodecomprising the conductive element; (b) determining at least one of thecapacitance and impedance characteristics of the body tissue byassessing the response to the applied electrical signal; and (c)determining whether the conductive element of the probe is locatedwithin the body tissue of at least one cancellous bone, cortical bone,and cortical bone near a boundary with soft tissue based on at least oneof the capacitance and impedance characteristics of the body tissue. 2.The method of claim 1, wherein the determined characteristics of thebody tissue include a phase angle relationship between a voltage andcurrent of the applied electrical signal and the response to the appliedelectrical signal.
 3. The method of claim 1, wherein the probe is apedicle probe.
 4. The method of claim 1, wherein the applied electricalsignal has a range of predetermined frequencies.
 5. The method of claim4, wherein the range of predetermined frequencies is from 400 KHz to 100MHz.
 6. The method of claim 1, wherein the probe is a pedicle screw. 7.The method of claim 1, wherein the applied electrical signal has asliding frequency.
 8. An article of manufacture for use in determiningwhether a conductive element of a probe is located within body tissue ofat least one cancellous bone, cortical bone, and cortical bone near aboundary with soft tissue, the article of manufacture comprisingcomputer readable storage media including program logic embedded thereinthat causes control circuitry to perform the steps: (a) applying anelectrical signal to a bi-polar electrode comprising the conductiveelement; (b) determining at least one of the capacitance and impedancecharacteristics of the body tissue by assessing the response to theapplied electrical signal; and (c) determining whether the conductiveelement of the probe is located within the body tissue of at least oneof cancellous bone, cortical bone, and cortical bone near a boundarywith soft tissue based on at least one of the capacitance and impedancecharacteristics of the body tissue.
 9. The article of manufacture ofclaim 8, wherein the determined characteristics of the body tissuefurther include a phase angle relationship between a voltage and currentof the applied electrical signal and the response to the appliedelectrical signal.
 10. The article of manufacture of claim 8, whereinthe probe is a pedicle probe.
 11. The article of manufacture of claim 8,wherein the applied electrical signal has a range of predeterminedfrequencies.
 12. The article of manufacture of claim 11, wherein therange of predetermined frequencies is from 400 KHz to 100 MHz.
 13. Thearticle of manufacture of claim 8, wherein the probe is a pedicle screw.14. The article of manufacture of claim 8, wherein the appliedelectrical signal has a sliding frequency.
 15. An apparatus for use indetermining whether a conductive element of a probe is located withinbody tissue of at least one cancellous bone, cortical bone, and corticalbone near a boundary with soft tissue, the apparatus including: (a)means for applying an electrical signal to a bi-polar electrodecomprising the conductive element; (b) means for determining least oneof the capacitance and impedance characteristics of the body tissue byassessing the response to the applied electrical signal; and (c) meansfor determining whether the conductive element of the probe is locatedwithin the body tissue of at least one of the cancellous bone, corticalbone, and cortical bone near a boundary with soft tissue based on atleast one of the capacitance and impedance characteristics of the bodytissue.
 16. The apparatus of claim 15, wherein the determinedcharacteristics of the body tissue include a phase angle relationshipbetween a voltage and current of the applied electrical signal and theresponse to the applied electrical signal.
 17. The apparatus of claim15, wherein means for applying an electrical signal includes means forapplying an electrical signal having a range of predeterminedfrequencies.
 18. The apparatus of claim 17, wherein the range ofpredetermined frequencies is from 400 KHz to 100 MHz.
 19. The apparatusof claim 15, wherein the probe is a pedicle screw.
 20. The apparatus ofclaim 15, wherein the applied electrical signal has a sliding frequency.21. The apparatus of claim 15, wherein the probe is a pedicle probe.